No Guts, No Glory

Capturing prey – no matter how spectacularly – is not, of itself,
enough. Neither is merely swallowing it. An animal – such as a shark or a
human – with a two-opening gut (with a mouth at one end and a cloaca or
anus at the other) can be thought of as a tube. By this analogy, a fish or
seal in a White Shark's stomach is no more inside the shark than a finger
inserted through the hole of a doughnut is inside the doughnut (those of
you more health-conscious may substitute a bagel for the doughnut in this
example – the analogy holds). Somehow, food surrounded by the shark's
stomach must be passed into the shark itself. Therefore, how the White
Shark processes and absorbs a food item is thus vitally important to its
ability to make use of the caloric energy it contains.

Gastrointestinal Tract of a
White SharkRedrawn from Parker (1887)

Like that of other sharks, the
esophagus of the Great White is
relatively short and lined with finger-like extensions that help prevent
food escaping out the mouth. In the floor of the mouth is a small,
relatively immobile 'tongue' (actually a forward-projecting extension of
the cartilaginous supports at the midline of the gill 'basket' to which
the hyoid arches attach). Control of food moving into the White Shark's
stomach is accomplished by way of a contractile ring of muscle called a
sphincter. The stomach itself is a or J-shaped organ, constituting about
20% of the shark's body length, and located along the midline of the body
just posterior to the gills. The stomach walls are quite muscular, the
inner lining having longitudinal folds called "rugae"
which permit accordion-like
expansion of the stomach to accommodate a particularly large meal. The
maximum capacity of a White Shark's stomach is about 10% of its total
weight, an ability which must come in handy for an opportunistic predator.
In addition, the stomach lining is peppered with various secretory
cells.
Some of these cells produce hydrochloric acid (which helps soften and
break-down bone), others copious amounts of mucus (to protect the stomach
lining from digesting itself). In addition, the pancreas (which produces
protein-splitting enzymes) has a duct that empties into the stomach,
thereby beginning the process of protein break-down.

The stomach wall itself is largely composed of transverse rings of
muscle, which – through a coordinated series of rhythmic squeezings
called "peristalsis" –
churn and mix the food and digestive secretions into a gloopy paste. The
control of food out of the stomach and into the intestine is governed by
another sphincter. Despite all its destructive machinery, the shark
stomach can be used to store food for prolonged periods (on the order of
months), somehow over-riding the stomach-distention sensor that would
otherwise initiate the stomach's food break-down processes. As in humans,
indigestible items can be voided out the mouth. But, unlike ourselves, at
least some sharks can evert the stomach at will: turning it inside-out
through the mouth, rinsing the stomach lining in sea water, then
retracting and returning it to the normal inside-in condition. Although no
one has yet reported this behavior in the Great White, sharks can
apparently do this all about as easily as you or I might return an
inside-out sock to its correct topology.

Another shark anatomical peculiarity is the liver. The liver of a shark
is composed of two large lobes, left and right, and a much smaller median
lobe. Unlike the dark chocolate-brown liver of the Spiny Dogfish (Squalus
acanthias) – a creature which many of us dissected as part of our
formal education in high school and/or university – the liver of the White
Shark is typically a pale orangy-peach in color. Compared with other
animals, the liver of sharks is very large – typically accounting for 15
to 25% of the total body weight, and up to 35% of the total weight in some
mesopelagic (mid-water) squaloids. Compare this with an average liver
weight of about 1.5 to 2.2% of total body weight in humans. So, what do
sharks in general – and Great Whites in particular – do with all that
liver?

The vertebrate liver is a remarkable chemical factory, performing
literally hundreds of roles in the body. Among many other functions, the
liver banks vitamins for release in times of low supply, manufactures a
starch-like compound that is used as a fuel supply by white muscle and can
be used by other tissues in an emergency, stabilizes the body's
blood-sugar level, detoxifies poisons, builds enzymes, processes digested
fats, manufactures bile and cholesterol, and constitutes a major source of
metabolic heat. In sharks, the liver is perfused with low-density oils and
hydrocarbons. One of the most important of these hydrocarbons is squalene
(C30H50),
which is much less dense than seawater. Because sharks lack a swim bladder
(an organ found in many teleosts that controls the overall density of the
fish by the controlled secretion and absorption of the gases within it),
sharks are heavier than water. The collective effect of the low-density
compounds in the shark liver is to provide lift by reducing its overall
density. As a result, a shark is only very slightly more dense than sea
water, making a 'typical' shark only slightly heavier than the medium
through which it swims. For example, an intriguing experiment by US Navy
researcher H. David Baldridge revealed that a 1015-pound (461-kilogram)
Tiger Shark (Galeocerdo cuvier) had an apparent weight in seawater
of only 7.3 pounds (3.3 kilograms) – a reduction of over 99%! Thanks
largely to its oily liver, a shark must invest very little energy to
prevent sinking and the vast majority of its swimming effort can go toward
propulsion.

The shark liver thus acts as a kind of internal float, freeing these
animals from depending solely on their foils to provide lift. It is widely
stated that the liver also serves sharks as an energy store, as it does in
most vertebrates – a position Baldridge very much doubts, given that the
last thing a starving shark should do is mess with its ability to swim.
However, in a 1987 paper, South African zoologist G.J. Rossouw reported
that the liver weight and oil content of the Lesser Guitarfish (Rhinobatos
annulatus) increase dramatically during periods of peak mating
activity. These changes could represent an energy store to fuel the
muscles for this species' annual migration, or it could simply be a way to
temporarily increase the Guitarfish's buoyancy while pupping or mating in
brackish water (which is less dense, and thus less buoyant, than
full-strength seawater). The White Shark has a relatively small liver by
shark standards, typically constituting 8 to 12% of the total body weight.
In general, there is an inverse relationship in sharks between activity
level and liver size. Thus the White Shark's relatively small liver is
consistent with its active lifestyle. This may mean that the White Shark
is more dependent upon dynamic lift than, for example, the slow-swimming
Basking Shark or a mesopelagic squaloid. Further, if there is any truth to
the idea that sharks draw on energy stored in the liver during lean times
(a notion for which there is some circumstantial evidence, but – to date
–
has only been conclusively demonstrated under conditions of extreme
starvation), then the Great White may be more tightly bound to areas
offering rich feeding than are other sharks.

In terms of energy per unit of mass, fats are rich food (which is why
extra calories stored in human 'love handles' are composed of fat rather
than protein: less weight to carry around). The relatively simple fats and
oils produced by fishes are fairly easy to break-down, but the more
complex fats that compose marine mammal blubber are much more difficult to
digest. The liver produces a thick yellowish-green liquid called bile,
which contains enzymes that emulsify fats. By reducing fat globules to
smaller droplets, bile increases the amount of surface area of each
droplet relative to its volume, thereby making it easier to digest. Bile
from the liver is stored in the gall bladder, a greenish sac located on
the underside of the median lobe. From there, it is transported (via a
duct) to the anterior part of the intestine. The intestine is the complex
tube at the posterior part of the vertebrate digestive tract where the
actual absorption of nutrients occurs. Only after broken-down nutrients
are passed through the intestinal wall are they inside the body and
available to fuel its many processes.

Cross-section
of the Intestine
of a White SharkRedrawn for Qingwen & Yuangding (1985)

In an average-sized adult human, the combined length of the small and
large intestine is about 26 feet (8 metres). Yet the intestine of a
comparably-sized shark is only about a foot (30 centimetres) in length.
Despite the intestine's shortness, it manages to absorb enough nutrients
to maintain the shark due to a system of internal partitions. Known as
intestinal valves, they are a triumph of compact packaging – greatly
increasing the internal surface area of the intestine without
significantly increasing its length. In addition, these partitions greatly
increase the efficiency of nutrient absorption even further by greatly
increasing the time required for food to pass through the intestine. For
example, shark biologist Brad Wetherbee and his co-workers used X-ray
technology to measure the of the rate of food passage through the stomach
and intestinal valve of juvenile Lemon Sharks fed on a uniform diet. They
found that the sharks began initial voiding of undigested matter some 16
to 17 hours after feeding and that to completely empty the digestive tract
required an astonishing 68 to 82 hours. In contrast, humans may process
food and excrete solid waste in as little as four hours after eating
(considerably less for certain spicy foods!)

There are three basic types of intestinal valve in sharks, termed
spiral, scroll, and ring. The spiral valve is the 'classic' shark
intestinal partition, resembling an auger in shape, and is found in cow
sharks (family Hexanchidae), spiny dogfishes (Squalidae), and catsharks
(Scyliorhinidae). The scroll valve resembles a loose roll of paper in
shape, and is found in whaler sharks (Carcharhinidae, such as the Lemon
Shark used in Wetherbee's gut-clearance experiment). The ring valve,
resembling a series of tightly-packed lamellae (plates), is found in all
extant lamnoids – including the White Shark. The ring valve offers the
most absorptive surface area per unit length, and is thus highly
efficient. This efficiency is in-keeping with the White Shark's
high-energy lifestyle. Further, the intestine in this species is typically
about 10% as long as its body, roughly 25% as wide as it is long, and has
from 47 to 54 lamellae ('rings'). For a 6-foot (2-metre) -long White
Shark, that works out to a total absorptive surface area of about 570 to
660 square inches (3,690 to 4,240 square centimetres), or about the same
as a Monopoly board. By comparison, the total surface area of the small
and large intestines of a 6-foot-tall man is about 495 square inches
(3,175 square centimetres) – actually some 25% less than a
comparably-sized White Shark.

But, just as a car produces exhaust, the White Shark does not use its
fuel with 100% efficiency. After its ring valve has removed as much
nutrient as possible from the stately parade of organic matter passing
through the intestine, what solid matter that remains is excreted as
waste. Because sharks do not have separate openings for products of the
digestive and reproductive systems, the posterior opening of the gut is
referred to as a cloaca instead of as an anus, as in humans. Rather than
producing a more-or-less solid bolus of waste matter, as do human anuses
(most of the time, anyway), shark cloacae void feces that is generally
quite liquidy, typically a dirty mustard brown in color. Sometimes, a
streaming cloud of shark feces is punctuated with an unexpectedly pretty
surprise: a burst of silvery fish scales, glittering like diamonds in the
flickering underwater light. With a graceful swish of its tail, the White
Shark dissipates the momentary discoloration, returning its unused energy
to the sea.